It is known technology to display the flight path angle of an aircraft on an aircraft attitude display indicator. Since flight path angle is the angle between the velocity vector of the aircraft and the horizon, it has been found useful to navigate the aircraft in a vertical direction by reference to a flight path angle symbol.
For example in U.S. Pat. No. 3,668,622 by Gannett et al there is disclosed an aircraft attitude display having superimposed thereon a first symbol indicating aircraft actual flight path angle and a second symbol indicating aircraft potential flight path angle.
A heads up display which has superimposed thereon a symbol representing the aircraft flight path is disclosed in U.S. Pat. No. 3,686,626 by Bateman et al.
In U.S. Pat. 2,896,145 by Snodgrass, flight path angle is determined by two different methods and the results combined to operate a flight path angle indicator. And, in U.S. Pat. No. 4,373,184 by Lambregts there is disclosed an attitude indicator on which the actual flight path angle of an aircraft and the commanded flight path angle of the aircraft are both displayed.
Furthermore, an aircraft flight path angle display system using a "quickened" flight path angle indicator is discussed in U.S Ser. No. 499,290 filed May 31, 1983, now abandoned, entitled "Aircraft Flight Path Angle Display System" by Lambregts and assigned to the assignee of the present invention.
A number of problems have arisen with conventional flight path angle displays. For example, in those displays which show both commanded flight path angle and actual flight path angle, there is sometimes a tendency for the pilot to be confused by the presence of both symbols. No intuitively clear symbology exists to indicate which symbol is commanded flight path angle and which symbol is actual flight path angle.
In some displays only one flight path angle symbol is shown. Typically this flight path angle symbol represents the actual flight path angle of the aircraft. In turbulent conditions or where sudden, abrupt changes are made to the direction of aircraft flight, the actual flight path angle symbol tends to be unstable thereby making it difficult to navigate the aircraft by reference to this symbol.
Another problem with navigating an aircraft by reference to the actual flight path angle symbol is due to the lag in the symbol movement when responding to changes in aircraft pitch. This lag is due to the fact that when the aircraft pitches up from level flight, for example, the center of gravity of the aircraft, and hence the aircraft velocity vector, initially is directed downward. This is a result of a downward force on the tail when the pilot pulls back on the control column to pitch the aircraft up. In this manner the initial value of flight path angle is negative until the aircraft begins climbing, and it accounts for the lag in the flight path angle display.